1. Field of the Invention
The present invention relates to a photoelectric converter and a radiation reader, more particularly to a photoelectric converter capable of performing the read operation at higher sensitivity and higher speed and a radiation reader capable of reading the information for radiation represented by α-rays, β-rays, γ-rays or x-rays at a high sensitivity.
2. Related Background Art
In the case of a photoelectric converter and a radiation reader for reading the information based on radiation by wavelength-converting the radiation into the sensitivity region of the photoelectric converter with a wavelength converter such as a photoluminescent body (for example, scintillator), electric charges based on the input information photoelectric-converted by a photoelectric-converting section is transferred to a capacitance to amplify a signal voltage.
However, to amplify a signal voltage by transferring electric charges from the capacitance of a photoelectric-conversion element itself to an external capacitance like the case of reading signal charges with a conventional circuit, a relatively-large S/N ratio can be obtained but a parasitic capacitance may be formed on a signal line when arranging a plurality of sensors. For example, when arranging 2,000×2,000 area sensors equivalent to an X-ray film at a size of 200×200 μm per cell and manufacturing an area sensor with a size of 40×40 cm, a capacitance is formed because the gate and source of a transistor for transferring electric charges are overlapped each other. Because the above overlap corresponds to the number of pixels, one signal line has an overlap Cgs capacitance of 0.05 pF×2,000 area sensors=100 pF even if one area sensor has a capacitance of approx. 0.05 pF. Because a sensor capacitance Cs is equal to approx. 1 pF, when assuming a signal voltage generated in a sensor as V1, the output voltage V0 of the signal line is obtained from the following expression.V0=(Cs/(Cs+Cgs×1000))×V1 Thus, the output voltage lowers to 1/100.
That is, to constitute an area sensor having a large area, an output voltage is greatly lowered.
Moreover, to read a dynamic image under the above condition, a sensitivity and a high-speed operation capable of reading 30 images per sec. or more are requested. Particularly, in the case of a non-destructive inspection including X-ray diagnosis, higher sensitivity allowing the number of signal charges to increase up to 100 to 400 times is requested because there is a request for minimizing the dose of X-rays to be irradiated.